In a typical simplified boiling water reactor (SBWR), a vacuum breaker valve is positioned between a drywell and a wetwell to facilitate proper operation of the passive containment cooling system (PCCS) of the reactor. Particularly, a hollow vent pipe extends through the floor of the drywell, and the vacuum breaker valve is coupled to the vent pipe and facilitates transporting particles, e.g., noncondensibles, between the drywell and the wetwell.
Known vacuum breaker valves are designed and tested to comply with various criteria and to demonstrate reliability for all credible events. For example, vacuum breaker valve leak tightness and reliability are important for successful reactor operation. Particularly, such leak tightness and reliability directly affect the pressure difference between the wetwell and the drywell, and the PCCS relies on such pressure difference to operate. More particularly, vacuum breaker valves limit drywell negative pressure, which reduces the possibility that negative drywell pressure might exceed the containment liner negative pressure service limit and substantially prevents the suppression pool water from rising to the level of the spillover holes in the main vents.
Known vacuum breaker valves typically are swing check valves which include a pivot pin pivotally coupled to a breaker valve body with bearings. The pivot pin also is coupled to a disk which is sized to cover one end of the vent pipe. An inlet screen is attached to the vent pipe and substantially prevents debris from flowing from the wetwell and through the vent pipe. In normal operation, the vacuum breakers provide a wetwell to drywell path for inerting nitrogen supplied by the Containment Atmospheric Control System (CACS). In addition, external forces, e.g., gravity, cause the pivot pin to rotate with the disk between a first, or closed, position, where the disk substantially seals one end of the standpipe, and a second, or open, position, where the end of the standpipe is at least partially unsealed. In the sealed position, the disk abuts a valve seal, or seat, which is positioned about a perimeter of the vent pipe, and the disk-seat-pipe connection is substantially leak tight. Gravity and positive drywell to wetwell pressure difference typically cause the disk to rotate to and remain in the closed position. However, if negative pressure difference exceeds a valve lift pressure, which typically is one half pounds per square inch (0.5 psid), then the disk rotates to the open position and enables noncondensibles to pass through the vent pipe between the wetwell and the drywell.
Vacuum breaker valve leakage, or failure to properly close and seal the vent pipe, possibly could result in degradation of heat removal capability during a loss of coolant accident (LOCA), which is undesirable. In addition, such failure to properly close and seal the vent pipe possibly could also cause suppression pool water to rise through the suppression pool vent pipes to the level of the spill over holes and uncover the PCCS vents and create a potential suppression pool bypass path during LOCA, which is undesirable.
Swing check valves, while generally acceptable, may possibly fail to close properly during reactor operation. While moving from the open position to the closed position, the closure force exerted between the disk and the vent pipe reduces, which is undesirable. If the closure force is insufficient, the swing check valve possibly may not close properly. Swing check valves also potentially are vulnerable to valve failure during reactor operation. For example, if debris passes through the inlet screen, such debris may affect the integrity of the breaker valve seat and bearing. In addition, swing check valves tend to chatter at low flow rates, and such chatter sometimes causes unnecessary valve seat and bearing wear, which is undesirable.
It would be desirable to provide a vacuum breaker valve assembly which exerts a more uniform closure force between the disk and the vent pipe and provides an improved seal, as compared to a swing check valve. It further would be desirable to provide such an assembly which is substantially simple to install in a reactor.